1. Field of the Invention
This invention relates to formation of semiconductor grade bulk HgCdTe.
2. Brief Description of the Prior Art
Precipitation of tellurium can occur during cooling of tellurium-rich HgCdTe. Tellurium precipitates form initially on dislocations and within the matrix. Dislocations act as heterogeneous nucleation sites for the formation of tellurium precipitates and precipitates may also form in the HgCdTe matrix between dislocations. When the bulk HgCdTe is subsequently annealed at low temperatures of about 270.degree. C. in a saturated Hg vapor to convert it from p-type (Te-rich) to n-type, a high dislocation density due to dislocation multiplication generally results. This defect multiplication results from a volume expansion at tellurium precipitates on dislocations when interstitial mercury combines with the tellurium to form HgTe according to the equation: EQU Te(precipitate)+Hg (interstitials).fwdarw.HgTe (plus lattice expansion).
Since the excess volume must be accommodated, the lattice is plastically deformed by this excess volume and expands to create more defects. See, for example the article of H. F. Schaake et al., Journal of Electronic Materials, Vol. 6, page 931 (1983).
Dislocations are known to be electrically active and can contribute to increased recombination in HgCdTe and dark currents in MIS devices, however they have also been blamed for having a negative influence on the electrical and other characteristics of semiconductor devices and degrade performance thereof. A discussion of this is set forth in "A Discussion of the Impact of Dislocations on Electrical Properties of HgCdTe", J. H. Tregilgas et al., Journal of Crystal Growth, 86 (1988), pp 460-466.
A prior method for circumventing dislocation multiplication in bulk Te-saturated HgCdTe is set forth in U.S. Pat. No. 4,481,044 of Schaake et al. wherein the amount of tellurium on the dislocations is reduced by high temperature annealing at about 600.degree. C. in a mercury saturated ambient. Removal of tellurium on dislocations prior to low temperature annealing in mercury vapor prevents dislocation multiplication. In accordance with the procedure in this patent, a slice or ingot of HgCdTe is placed in one side of an enclosed ampoule and mercury is placed at the other end of the ampoule. The temperature at the HgCdTe end of the ampoule is maintained at about 600.degree. C. whereas the temperature at the mercury end of the ampoule is maintained at about 550.degree. C. to anneal the HgCdTe to allow mercury to enter into the HgCdTe and change the composition thereof. The HgCdTe is then post-annealed below 325.degree. C. in a mercury saturated atmosphere and allows additional mercury to enter therein to further change the composition of the HgCdTe. The result is n-type HgCdTe whereas the starting material is filled with metal vacancies. This method does not provide suitable results when used to reduce dislocations in thin films, such as those formed by liquid phase epitaxy (LPE) from Te-rich solutions.
A common problem encountered in slice dislocation reduction annealing (DRA) processing of bulk HgCdTe and liquid phase epitaxially (LPE) deposited HgCdTe films is that dislocation multiplication can occur during heating from room temperature in the presence of liquid mercury if tellurium precipitates are present at dislocation lines. This problem has been observed in bulk material by a graded dislocation density near the surface of the slice extending several mils into the material as noted in the above Tregilgas article. In the case of LPE grown films which are typically less than 150 micrometers in thickness, dislocation multiplication may occur extensively throughout the film, raising the dislocation density by a factor of two or more.
It is desired to find a procedure whereby the defect multiplication problem of the prior art can be circumvented in thin films such as those formed by LPE without the problems encountered in the prior art.